A-site-and/or b-site-modified pbzrtio3 materials and (pb, sr, ca, ba, mg) (zr, ti,nb, ta)o3 films having utility in ferroelectric random access memories and high performance thin film microactuators

ABSTRACT

A modified PbZrTiO 3  perovskite crystal material thin film, wherein the PbZrTiO 3  perovskite crystal material includes crystal lattice A-sites and B-sites at least one of which is modified by the presence of a substituent selected from the group consisting of (i) A-site substituents consisting of Sr, Ca, Ba and Mg, and (ii) B-site substituents selected from the group consisting of Nb and Ta. The perovskite crystal thin film material may be formed by liquid delivery MOCVD from metalorganic precursors of the metal components of the thin film, to form PZT and PSZT, and other piezoelectric and ferroelectric thin film materials. The thin films of the invention have utility in non-volatile ferroelectric memory devices (NV-FeRAMs), and in microelectromechanical systems (MEMS) as sensor and/or actuator elements, e.g., high speed digital system actuators requiring low input power levels.

This is a division of U.S. application Ser. No. 09/026,946 filed Feb.20, 1998, now U.S. Pat. No. 6,312,816.

GOVERNMENT RIGHTS IN INVENTION

The invention disclosed herein was made in the performance of UnitedStates Air Force Contract No. F33615-95-C-5428 (PSZT aspect) and NASAContract No. NAS8-97176 (PNZT aspect). The government has certain rightsin the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the chemical vapor deposition (CVD)formation of Pb(Zr,Ti)O₃ materials modified with Group II cations (Sr,Ca, Ba and/or Mg) on the A-sites thereof, and Nb and/or Ta on theB-sites thereof, and to (Pb, Sr, Ca, Ba, Mg)(Zr, Ti, Nb, Ta)O₃ filmshaving utility in ferroelectric random access memories, high performancethin film microactuators, and in related device applications.

2. Description of the Related Art

Ferroelectric materials are presently finding increased application indevices including non-volatile ferroelectric random access memories(NV-FeRAMs), uncooled infrared (IR) detectors, spatial light modulators,and microelectromechanical systems (MEMS). Many of these applicationsrequire optimized ferroelectric, pyroelectric and related properties,which are known to be sensitive to film compositions and incorporationsof dopants or modifiers.

In order to effect such compositional variation, there is a need in theart for corresponding processes enabling the production of perovskitefilms with superior compositional and performance properties.

Directing the discussion now to a relevant background aspect of thepresent invention, the development of reliable actuation methods anddevices is one of the key challenges in the evolution from micromachinedsensors to microelectromechanical systems (MEMS). High qualitypiezoelectric films possess numerous properties of technologicalimportance for such MEMS applications, including high electromechanicalcoupling coefficients and high piezoelectric coefficients. The mostcommon family of material exhibiting both of these characteristics arebased on Pb(Zr_(y)Ti_(1−y))O₃ (PZT).

One primary factor limiting development of piezoelectric MEMS has beenthe lack of suitable, high quality thin film piezoelectric materials.PZT and related compositions are the best piezoelectric materialsavailable in bulk form and are a logical choice for thin-filmmicroactuator applications.

A number of microactuator devices can be envisioned that are based oncantilever-type deflection, including optical devices and liquid controldevices. Depending upon the application, the requirements for operatingdeflection needed in such devices may vary widely. In a cantileveredpiezoelectric microactuator of the type that may be usefully employed inpositioners and microvalves, the achievable deflection for an appliedvoltage is directly proportional to the piezoelectric coefficient d₃₁.Since the deflection is directly proportional to the applied voltage andto the piezoelectric coefficient d₃₁, an increase in d₃₁ at a givenvoltage increases the deflection. Looking at this relationship anotherway, for a given deflection, the drive voltage is reduced with increasedd₃₁. Lower drive voltage requirements are also a significant advantageas integration of PZT microactuators into integrated circuit (IC)devices is contemplated by the art and this remains important even forsmall displacement devices.

Accordingly, it would be a substantial advance in the art to developcompositions and process technology for the achievement of high qualityfilms that are able to maximize deflection for a given drive voltage inmicroactuator applications. Similarly, these high quality films may haveother advantageous properties.

Integration of thin film PZT and related materials into MEMSapplications requires a well-controlled process that affords precisecontrol of composition to maintain acceptable device performance acrossa wafer and from wafer-to-wafer. In addition, good step coverage isrequired for micromachining of the devices to protect the edges offeatures from unwanted etching. Finally, the process must be highlyeconomical. This last requirement is comprised of several factorsincluding the ability to process large area Si wafers and achieve highprocess throughput. Although the state of the art of bulk piezoelectricceramic materials has changed little in the last decade, considerableeffort has been focused on techniques to produce thin films of PZT andrelated materials.

RF sputtering (“Epitaxial Growth and Electrical Properties ofFerroelectric Pb(Zr_(0.9)Ti_(0.1))O₃ Films by Reactive Sputtering,” T.Okamura, M. Adachi, T. Shiosaki, A. Kawabata, Jap. J. Appl. Phys 30-1(1991): 1034), sol-gel formation (“Low Temperature Perovskite Formationof Lead Zirconate Titanate Thin Films by a Seeding Process,” C. K. Wokand S. B Desu, J. Mater. Res. 8 (1993): 339), and CVD (“Preparation andProperties of (Pb,La)(Zr,Ti)O₃ Thin Films by Metalorganic Chemical VaporDeposition,” M. Okada and K. Tominaga, J. Appl. Phys. 71 (1992): 1955;and “Growth and Characterization of Ferroelectric Pb(Zr,Ti)O₃ Thin Filmsby MOCVD Using a 6 Inch Single Wafer CVD System,” M. Shimizu, M.Fujimoto, T. Katayama, T. Shiosaki, K. Nakaya, M. Fukagawa, and E.Tanikawa, ISIF'93 Proceedings, Colorado, Springs, Colo. (1993)) have allbeen used to make high quality thin film PZT.

RF sputtering is an inherently low deposition rate process for complexoxide materials like PZT and uniform composition is difficult to achieveacross large areas. In addition, as sputtering targets wear, compositioncan drift and cross-target contamination is extremely problematic forprocess control. Sol-gel processes offer better control of composition,but have poor step coverage. Moreover, sol-gel processing of PZTrequires post-deposition annealing, which can lead to vaporization andloss of Pb, and can affect underlying IC structures. Although progresshas been made in lowering processing temperatures, for example by theuse of seed layers (“Low Temperature Perovskite Formation of LeadZirconate Titanate Thin Films by a Seeding Process,” C. K. Kwok and S.B. Desu, J. Mat. Res. 8 (1993): 339), these temperatures are stillhigher than those which have used with success to deposit PZT by MOCVDtechniques of the prior art.

Therefore, a process is desired for the formation of thin films of PZTand related materials, which affords compositional control, providesuniformity of the thin film material over large areas, and achieves ahigh degree of conformality on the substrate structure, as well as ahigh deposition rate. The deposited material should also be free ofpinholes, since in capacitive and many other devices, pinholes willresult in a shorted, useless device.

For thin film PZT and related materials, precise and repeatablecompositional control is required in order to produce films of highquality. Physical deposition methods (e.g., sputtering, evaporation) ofthin film deposition are deficient in this regard, as are traditionalapproaches to MOCVD involving the use of bubblers.

Turning to ferroelectric PZT, it is generally recognized that many ofthe electrical properties can be improved by replacing A or B sitespecies with cations of a higher oxidation state. This is typicallyreferred to as donor doping. In specific cases improvements in leakageresistance, fatigue and imprint have been attributed to donor doping.Improved leakage resistance is observed for donor doping and is believedto be a result of compensation of native and impurity acceptor defects.Improvements in fatigue have been reported for doping with yttrium (Y),(Kim, J. H.//Paik, D. S.//Park, C. Y.//Kim, T. S.//Yoon, S. J.//Kim, H.J.//Jeong, H. J., ‘Effect of Yttrium Doping on the Ferroelectric Fatigueand Switching Characteristics of Pb(Zr_(0.65)Ti_(0.35))O₃ Thin-FilmsPrepared by Sol-Gel Processing’, INTEGRATED FERROELECTRICS, (10), 1995,pp. 181-188), lanthanum (La), (Shimizu, M.//Fujisawa, H.//Shiosaki, T.,‘Effects of La and Nb Modification on the Electrical-Properties ofPb(Zr,Ti)O₃ Thin-Films by MOCVD’, INTEGRATED FERROELECTRICS, 14, 1997,pp. 69-75), niobium (Nb), (Tuttle, B. A.//Alshareef, H. N.//Warren, W.L.//Raymond, M. V.//Headley, T. J.//Voigt, J. A.//Evans, J.//Ramesh, R.,‘La_(0.5)Sr_(0.5)CoO₃ Electrode Technology for Pb(Zr,Ti)O₃ Thin-FilmNonvolatile Memories’, MICROELECTRONIC ENGINEERING, 29, 1995, pp.223-230.), and tantalum (Ta), (Choi, G. P.//Ahn, J. H.//Lee, W.J.//Sung, T. H.//Kim, H. G., ‘Phase Formations and Electrical-Propertiesof Doped-PZT/PbTiO3 Films Deposited by Reactive Sputtering UsingMulti-Targets’, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATEMATERIALS FOR ADVANCED TECHNOLOGY, 41, (1), 1996, pp. 16-22.).Significantly enhanced imprint resistance was demonstrated by donordoping with Ta using a sol-gel thin film fabrication process (W. L.Warren, D. Dimos, G. Pike, B. Tuttle, and M. Raymond, “Voltage shiftsand imprint in ferroelectric capacitors”, Appl. Phys. Lett., 67 (6),(1995), pp. 866-868.).

Doping is straightforwardly achieved using deposition processes such assputtering and sol-gel. To achieve doped PZT by MOCVD requiresidentification of suitable precursor chemicals that decompose to thedesired product and do not undergo undesirable interactions duringdelivery and transport to the substrate. Few examples of doping by MOCVDexist in the prior art, and as will be described, many have never beendescribed to date. The most common dopant, La, has been deposited byMOCVD to provide PLZT films as reported by Van Buskirk, P. C.//Roeder,J. F.//Bilodeau, S., ‘Manufacturing of Perovskite Thin-Films UsingLiquid Delivery MOCVD’, INTEGRATED FERROELECTRICS, (10), 1995, pp.9-22.). While Nb doped PZT has been demonstrated by Shimizu et al., theyused tetraethyl Pb, Zr(tertiary t-butoxide)₄ and Ti(isoproxide)₄ for PbZr and Ti precursors, respectively, and Nb(ethoxide)₅ as a Nb precursor(Shimizu, M.//Fujisawa, H.//Shiosaki, T., ‘Effects of La and NbModification on the Electrical-Properties of Pb(Zr,Ti)O-3 Thin-Films byMOCVD’, INTEGRATED FERROELECTRICS, 14, 1997, pp. 69-75). Thedisadvantage of this approach is that tetraethyl Pb is toxic andrelatively hazardous due to its high vapor pressure at room temperatureand the lack of a suitable sensor to warn of its presence. Furthermore,the Nb precursor Nb(ethoxide)₅ is not compatible with safer Pbprecursors, such as a Pb(tetramethylheptandionate)₂, used in the presentapplication. It must be emphasized again that the discovery of acompatible set of well-behaved precursors for each doping application isessential.

There is therefore a pressing need in the art for new approaches to thedeposition of novel thin film materials of such type, and fornext-generation piezoelectric and ferroelectric materials applications.

SUMMARY OF THE INVENTION

The present invention relates in one aspect to a method to deposit thinfilm piezoelectric materials by MOCVD utilizing a liquid deliverytechnique. This technique affords precise compositional control byvirtue of mixing liquid precursor solutions and flash vaporization ofsame. Flash vaporization has the added benefit of preventing unwantedpremature decomposition of the precursor species; this is especiallyimportant for Group II metals (e.g., Sr, Ba, Ca and Mg). In addition,tailored precursor chemistries may be employed that are compatible foreach thin film material because they do not undergo ligand exchange (orligand exchange is degenerate). This approach prevents the formation ofinvolatile species and facilitates reproducible gas-phase transport ofthe reactants.

The present invention also relates to piezoelectric and ferroelectricthin film modified PZT materials, and to devices based thereon.

As used herein, the term “thin film” refers to a film having a thicknessof less than 200 μm.

Devices within the broad scope of the invention include, but are notlimited to, those utilizing the thin film ferroelectric modified PZTmaterials in piezoelectric actuating elements; in passive as well asactive MEMS devices; in optical devices, including both geometric andspectral-(or interference-) based devices, such as movable microlensarrays, or movable micromirror arrays, or in spectral devices to alter aresonant cavity in an etalon structure to detune the reflectance of thedevice; in micropumps and microvalves based upon a cantilever geometryof the piezoelectric film; for applications such as delivering doses ofmedication, running hydraulic or fluid flow systems in a MEMSconfiguration; in ultrasonic transducers and active vibration controldevices; in ultrasonic transducers for high frequency applicationsallowing spatial resolution for detecting small defects such as nearsurface flaws in aging aircraft; in microelectronics; and in biologicalapplications, as well as in uncooled infrared radiation pyroelectricdetectors; and in non-volatile ferroelectric memory devices, forapplications such as data storage (FeRAMs) and replacement of EEPROMsand flash memory.

In a specific aspect, the present invention relates to a modifiedPb(Zr,Ti)O₃ perovskite crystal material thin film, wherein thePb(Zr,Ti)O₃ perovskite crystal material includes crystal lattice A-sitesand B-sites at least one of which is modified by the presence of asubstituent selected from the group consisting of A-site substituentsconsisting of Sr, Ca, Ba and Mg, and B-site substituents selected fromthe group consisting of Nb and Ta.

Another specific compositional aspect of the invention relates to thinfilm (Pb,Sr)(Zr,Ti)O₃ (“PSZT”), e.g., thin film piezoelectric PSZT, andthin film ferroelectric PSZT.

A further specific aspect of the invention relates to amicroelectromechanical apparatus comprising a thin film piezoelectricPSZT element as a sensor and/or actuator element thereof.

Yet another aspect of the invention relates to a method of forming on asubstrate a modified Pb(Zr,Ti)O₃ perovskite crystal material thin film,wherein the Pb(Zr,Ti)O₃ perovskite crystal material includes crystallattice A-sites and B-sites at least one that is modified by thepresence of a substituent selected from the group consisting of A-sitesubstituents consisting of Sr, Ca, Ba and Mg, and B-site substituentsselected from the group consisting of Nb and Ta, comprising liquiddelivery MOCVD of the thin film from metalorganic precursors of themetal components of the thin film.

The metalorganic precursors may for example comprise metal(β-diketonates), such as titaniumbis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate) as a Tiprecursor; zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate) asa Zr precursor; zirconium bis(isopropoxide) bis(2,2,6,6-tetramethyl-3,5-heptanedionate) as a Zr precursor; leadbis(2,2,6,6-tetramethyl-3,5-heptanedionate) as a Pb precursor; strontiumbis(2,2,6,6-tetramethyl-3,5-heptanedionate).L, where L=a Lewis base, asa Sr precursor; niobium tetrakis(isopropoxide)(2,2,6,6-tetramethyl-3,5-heptanedionate) as a Nb precursor, and tantalumtetrakis(isopropoxide) (2,2,6,6-tetramethyl-3,5-heptanedionate) as a Taprecursor.

The modified Pb(Zr,Ti)O₃ perovskite crystal material thin film maysuitably have any appropriate stoichiometry and elemental composition.Illustrative modified PZT materials include:

(Pb,Sr)(Zr,Ti)O₃,

(Pb,Ca)(Zr,Ti)O₃,

(Pb,Ba)(Zr,Ti)O₃,

(Pb)(Nb,Zr,Ti)O₃,

(Pb)(Ta,Zr,Ti)O₃,

(Pb,Ca)(Ta,Zr,Ti)O₃,

(Pb,Sr)(Ta,Zr,Ti)O₃,

(Pb,Ca)(Nb,Zr,Ti)O₃, and

(Pb,Sr)(Nb,Zr,Ti)O₃.

In a specific aspect, the Pb(Zr,Ti)O₃ perovskite crystal material maycomprise a composition of the formula

Pb_(x)Sr_((1−x))Zr_(y)Ti_((1−y))O₃,

wherein Pb:Sr:Zr:Ti has a ratio x:(1−x):y:(1−y),

where x has a value of from about 0.86 to about 0.93, and

y has a value of from about 0.50 to about 0.60.

In another specific aspect, the Pb(Zr,Ti)O₃ perovskite crystal materialmay comprise a composition of the formula

Pb_((1−x/2))Nb_(x)[Zr_(y)Ti_((1−y))]_((1−x))O₃,

wherein Pb:Nb:Zr:Ti has a ratio (1−x/2):x:y(1−x):(1−y)(1−x),

where x has a value of from about 0.01 to about 0.07, and y has a valueof from about 0.40 to about 0.60.

In yet another specific aspect, the Pb(Zr,Ti)O₃ perovskite crystalmaterial may comprise a composition of the formula

Pb_((1−x))Ca_(x)[Zr_((y−z/2))Ti_((1−y−z/2))Ta_((z))]_((1−x))O₃,

wherein Pb:Ca:Zr:Ti:Ta has a ratio (1−x):x:(y−z/2)(1−x):(1−y−z/2)(1−x):z(1−x),

where x has a value from about 0.01 to about 0.05, y has a value of fromabout 0.40 to about 0.60, and z has a value from about 0.001-0.02.

Other aspects, features and embodiments of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a crystal lattice orientation of aperovskite composition of the formula ABO₃ wherein the A sites arepredominantly occupied by Pb and sparingly occupied by one or morespecies selected from the group consisting of Sr, Ba, Ca, and Mg, and/orwherein the B sites are predominantly occupied by Zr and Ti andsparingly occupied by at least one of the species selected from thegroup consisting of Nb and Ta. The isovalent substitutions are commonlyreferred to as “substitutionally modified PZT”, herein described for theA-sites, while the non-isovalent substitutions are commonly referred toas “dopants”, herein described for the B-sites. It is recognized thatother substitutional modifications and dopants may advantageously bemade for both A and B-sites.

FIG. 2 is a schematic diagram of a cantilevered piezoelectricmicroactuator, of a type which may be used in positioners andmicrovalves wherein achievable deflection for an applied voltage isdirectly proportional to the piezoelectric coefficient d₃₁.

FIG. 3 is a schematic representation of a liquid delivery system forsimultaneous, controlled introduction of metalorganic compounds to a CVDreactor.

FIG. 4 is a schematic representation of a CVD reactor that may beusefully employed for PZT film growth.

FIG. 5 shows the composition of unmodified PZT films deposited inaccordance with the method of the invention, in the region near themorphotropic phase boundary between rhombohedral and tetragonal PZT.

FIG. 6 shows the compositions of PSZT films deposited in accordance withthe method of the invention, with compositions at the room temperaturemorphotropic boundary being indicated. The phase boundary shifts towardthe PbZrO3 side of the system as Sr increases.

FIG. 7 shows representative X-ray diffraction patterns for Sr-modifiedPZT films in the composition range from Zr/Ti=44/56 to 54/46, whereinall films show similar mixed [100]+[111] preferred orientation.

FIG. 8 shows XRD patterns of PSZT films deposited by MOCVD in accordancewith the method of the invention.

FIG. 9 shows small signal AC capacitance as a function of applied DCbias for a PZT film deposited by MOCVD, wherein electrode area was8×10⁻⁴ cm², and film thickness was 5193 Å, and wherein the shift in themaxima with direction of bias sweep is characteristic of a ferroelectricfilm.

FIG. 10 shows the loss tangent as a function of composition for filmsdeposited by MOCVD in accordance with the method of the invention.

FIG. 11a shows ferroelectric hysteresis loops for representative filmsdeposited by MOCVD in accordance with the method of the invention,wherein Zr/Ti ratios are indicated along with Sr content, demonstratingthe ferroelectric effect in both PZT and PSZT.

FIG. 11b shows ferroelectric hysteresis loops for representative filmsdeposited by MOCVD in accordance with the method of the invention,wherein Zr/Ti ratios are indicated along with nominal Ca and Ta content,demonstrating the ferroelectric effect in PCTZT.

FIG. 12 shows a fundamental resonance vibrational mode of a cantileverbeam, wherein the horizontal axis corresponds to excitation frequencyfor a sinusoidal AC wave of ±2.8 V, and the vertical axis isproportional to the amplitude of the vibration, demonstrating thepiezoelectric effect in films produced by the method of the presentinvention.

FIG. 13 shows ferroelectric hysteresis loops for films deposited byMOCVD in accordance with the method of the invention, with a Zr/Ti ratioof 40/60 both with and without 1% Ca and 0.2% Ta substitution. Bothfilms show full saturation below 2.5V and high remanent polarization.

FIG. 14 shows the leakage current versus voltage for PZT with Zr/Ti of40/60 and the same Zr/Ti with 1% Ca and 0.2% Ta substitution. Thesubstituted material has {fraction (1/7)}th the leakage current densityat 3V compared to material which was not substituted.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

The present invention provides a commercially useful process for formingactive piezoelectric films having utility in microelectromechanicaldevices and ferroelectric random access memories, as well as in otherpiezoelectric and ferroelectric devices and applications. The process ofthe present invention is scalable to large areas for Si integration, andprovides new thin film materials in the Pb(ZrTi)O₃ (PZT) family ofpiezoelectrics that have the ability to significantly enhance thepiezoelectric effect, and other ferroelectric properties.

By way of discussion of the compositional aspects of the presentinvention, FIG. 1 is a perspective view of a crystal lattice orientationof a perovskite composition of the formula ABO₃ wherein the A sites arepredominantly occupied by Pb and sparingly occupied by one or morespecies selected from the group consisting of Sr, Ba, Ca, and Mg, and/orwherein the B sites are predominantly occupied by Zr and Ti andsparingly occupied by at least one of the species selected from thegroup consisting of Nb and Ta.

In the drawing, the A-sites are indicated by the atoms denoted by filledcircles “” and the B-sites are indicated by the centrally positionedatom denoted by empty circles “◯.” The oxygen atoms occupy the facecenters.

The amount of dopant(s) or modifying substituent(s) in the crystallattice may be varied in the broad practice of the present invention, toachieve desired orientation, structural, chemical, electrical andelectromechanical properties. In general, the amounts of each of theindividual modifying A-site and B-site substituents will not exceedabout 20 atomic percent of the crystal lattice.

Thus, (Pb, Sr, Ca, Ba, Mg)(Zr, Ti, Nb, Ta)O₃ compositions of the presentinvention may for example incorporate Sr as one of the A-sitesubstituents, to form ceramic compositions such as (Pb,Sr)(Zr,Ti)O₃(“PSZT”).

When strontium is employed as a modifying substituent in the PZT crystallattice, it may be utilized at any suitable concentration, but isgenerally in the range of from about 1 to about 15 atomic % of thecrystalline material. Such concentration range is analogous to thatusefully employed in bulk ceramic PZT, where substituent amounts of suchmagnitude dramatically increase d₃₁ by up to 60% (see FerroelectricMaterials and Their Applications, Y. Xu, New York: North Holland (1991):131 for a tabulation of dielectric constant, tan δ, kp, and d_(31,) aswell as F. Kulcsar, J. Am. Ceram. Soc. 42 (1959): 49 and 143). Suchimprovements of electrical properties in microactuator applicationspermit microactuator drive voltages to be reduced by levels on the orderof 40% of the drive voltages required by PZT per se.

The (Pb, Sr, Ca, Ba, Mg)(Zr, Ti, Nb, Ta)O₃ compositions of the inventionmay for example incorporate niobium as one of the B-site substituents,such as Pb(Nb,Zr,Ti)O₃ (PNZT). In these compositions, Nb dopant atomssubstitute for B-site atoms (namely Zr or Ti) in the ABO₃ perovskitelattice and they act as donors-.

When niobium is employed as a dopant in the PZT crystal lattice, it maybe utilized at any suitable concentration, but is generally in the rangeof from about 1 to about 10 atomic % of the crystalline material. Suchconcentration range is analogous to that usefully employed in bulkceramic PZT, where substituent amounts of such magnitude dramaticallyincrease the piezoelectric coefficient d₃₁ by up to 80% and theelectromechanical coupling factor kp by 30%. In addition, niobium dopinghas been found to drastically reduce electrical leakage by more than twoorders of magnitude and improve electrical breakdown strength, relativeto PZT per se. In ferroelectric memory applications, niobium doping ofPZT has been demonstrated to minimize imprint and coercivity whilemaximizing the remenance ratio (namely squareness in hysteresis loop),relative to PZT per se.

In accordance with the invention, the metalorganic precursors of thecomponent metals of the desired (Pb, Sr, Ca, Ba, Mg)(Zr, Ti, Nb, Ta)O₃film are introduced in liquid form, either as neat liquids or dilutesolutions if the precursor is a liquid at ambient temperature andpressure (e.g., 25° C. and atmospheric pressure) conditions, or if theprecursor composition is a solid at such ambient conditions, then as asolution of the precursor in a compatible liquid or solvent medium. Theliquid or solvent medium may be of any suitable type which is compatiblewith the specific precursor composition employed, as is known andunderstood by those skilled in the art of liquid delivery MOCVD.

The liquid precursor composition then is introduced into a vaporizationzone, in which the liquid is rapidly vaporized, e.g., by flashvaporization on a foraminous vaporization element heated to suitabletemperature, to produce a corresponding precursor vapor.

The precursor vapor then is transported to the chemical vapor depositionchamber, which may for example comprise a CVD reactor of known orconventional type, which may be suitably equipped to introduce theprecursor vapor for contact in the deposition chamber with a heatedsubstrate at a temperature which effects deposition of the metalconstituents of the vapor onto the substrate element. For this purpose,the substrate may be mounted on a heated susceptor or other substratemounting structure, with the spent vapor from the process beingdischarged from the deposition chamber and subjected to furthertreatment or processing in a known and conventional manner.

The liquid delivery method of the present invention may be employed toform (Pb, Sr, Ca, Ba, Mg)(Zr, Ti, Nb, Ta)O₃ films on any compatiblesubstrates, at any suitable thicknesses, and under any suitable processconditions in the liquid precursor vaporization and chemical vapordeposition steps. Among thin film deposition techniques, MOCVD offers aunique combination of composition control, uniformity over large areas,a high degree of conformality and a high deposition rate. Mostimportantly, MOCVD is capable of conformal film deposition onthree-dimensional structures, a crucial feature not present from anyother film formation techniques demonstrated to date. By using theliquid delivery MOCVD method it is possible to modify PZT withadditional element(s) in a straightforward manner to achieve the desiredproduct film characteristics.

The liquid delivery MOCVD method of the invention may be carried outwith introduction of the precursor reagents to the vaporization zone ofthe process system in combination with one another, or separately. Forexample, multiple vaporizers may be employed to vaporize differentprecursors in separate vaporization operations, with the resultingvapors of the separate operations then being blended for transport tothe chemical vapor deposition chamber. The precursor vapor(s) may bemixed with carrier gas(es), e.g., argon, helium, nitrogen, etc., as maybe necessary or desired in a given application.

It will be appreciated that the liquid delivery MOCVD method of theinvention may be carried out in any suitable manner, and the optimumprocess conditions may be readily determined without undueexperimentation by the skilled individual in the art, to achieve thedesired film on the substrate on which deposition is being conducted.

Further, the film as deposited may be further processed in any suitablemanner, e.g., by annealing according to a specific time/temperaturerelationship, and/or in a specific atmosphere or environment, to producethe final desired thin film form (Pb, Sr, Ca, Ba, Mg)(Zr, Ti, Nb, Ta)O₃material.

The thin film PSZT material of the invention has been heretoforeunknown, and is a novel composition of the present invention. Thepresent invention also embodies the first demonstration of d₃₁piezoelectric effects in PZT films deposited by metalorganic chemicalvapor deposition (MOCVD).

A wide variety of devices may be fabricated within the broad scope ofthe invention including thin film ferroelectric modified PZT materialsof the invention. For example, thin film structures of the modified PZTmaterials of the invention may be employed in piezoelectric actuatingelements, in passive as well as active MEMS devices, in optical devices,including both geometric and spectral (or interference) based devices,such as movable microlens arrays, or movable micromirror arrays. Filmsof such type may also find utility in spectral devices to alter aresonant cavity in an etalon structure to detune the reflectance of thedevice. Other potential uses of the thin film modified PZT materials ofthe invention include micropumps and microvalves based on cantilevergeometry of the piezoelectric film, for applications such as deliveringdoses of medication or running hydraulic or fluid flow systems in a MEMSconfiguration, ultrasonic transducers and active vibration controldevices, ultrasonic transducers for high frequency applications allowingspatial resolution for detecting small defects such as near surfaceflaws in applications such as evaluation of the structuralcharacteristics of aging aircraft, microelectronics, biologicalapplications, uncooled infrared radiation pyroelectric detectors, andnon-volatile ferroelectric memory devices for applications such as datastorage and replacement of EEPROMs and flash memory.

As a specific illustrative example of a device that may usefully employthe modified PZT thin film materials of the present invention, FIG. 2 isa schematic diagram of a cantilevered piezoelectric microactuator 10, ofa type which may be used in positioners and microvalves whereinachievable deflection for an applied voltage is directly proportional tothe piezoelectric coefficient d_(31.) The microactuator 10 includes acantilever beam member 12 comprising upper electrode 14, piezoelectricthin film element 16, lower electrode 18, and support membrane 20. Thecantilever beam member 12 as shown is positioned at one end on pedestal24 of the substrate 22 with the other end being positioned over actuatorcavity 26 to allow deflectional vertical movement of the cantilever beam12 in operation of the device, in the direction indicated bybi-directional arrow A.

A number of microactuator devices can be envisioned that are based oncantilever-type deflection, including optical devices and liquid controldevices. Depending on the application, the requirements for operatingdeflection needed in such devices may vary widely. In a cantileveredpiezoelectric microactuator of the type that may be usefully employed inpositioners and microvalves, the achievable deflection for an appliedvoltage is directly proportional to the piezoelectric coefficient d₃₁.Since the deflection is directly proportional to the applied voltage andto the piezoelectric coefficient d₃₁, an increase in d₃₁ at a givenvoltage increases the deflection. Looking at this relationship anotherway, for a given deflection, the drive voltage is reduced with increasedd₃₁. Lower drive voltage requirements are also a significant advantageas integration of microactuators into integrated circuit (IC) devices iscontemplated by the art and this remains important even for smalldisplacement devices. The modified PZT thin film materials of thepresent invention have piezoelectric coefficient characteristics thatrender them highly advantageous in such microactuator applications.

The metalorganic chemical vapor deposition process of the inventionpermits the production of films of PSZT enabling the development ofpiezoelectrically controlled microelectromechanical devices.Applications for such PSZT films are widespread. Optical uses range frommicromirrors to micro-etalons, which could be detuned from a reflectancecondition. Device dimensions from tens to hundreds of microns arepossible, and MOCVD has particular advantage in the fabrication ofarrays of devices where surface topology will be present. Other opticalpositioners can also be based on these films, such as deformablemirrors. In other device applications utilizing the thin film modifiedPZT materials of the invention, microvalves and micropumps have manyimportant applications from industrial control to gas sampling devices.

In one embodiment of the present invention, the precursors for the metalcomponents of the product film are dissolved in a solvent and flashvaporized at temperatures between about 100 to about 300° C. thermallyand carried into the MOCVD reactor with a carrier gas (e.g., Ar, N₂, He,or NH₄) where they are mixed with an oxidizing co-reactant gas (e.g.,O₂, N₂O, O₃) and transported to the deposition chamber to undergodecomposition at a substrate heated between about 400 to about 1200° C.at chamber pressures between about 0.1 to about 760 torr. Other activeoxidizing species may be used to reduce deposition temperature, asthrough the use of a remote plasma source.

Illustrative examples of compatible precursor chemistries are listedbelow, for production of corresponding (Pb, Sr, Ca, Ba, Mg)(Nb, Ta, Zr,Ti)O₃ films:

(Pb,Sr)(Zr,Ti)O₃,

(Pb,Ca)(Zr,Ti)O₃,

(Pb,Ba)(Zr,Ti)O₃,

(Pb,Mg)(Zr,Ti)O₃,

(Pb)(Nb,Zr,Ti)O₃,

(Pb)(Ta,Zr,Ti)O₃,

(Pb,Ca)(Ta,Zr,Ti)O₃,

(Pb,Sr)(Ta,Zr,Ti)O₃,

(Pb,Ca)(Nb,Zr,Ti)O₃, and

(Pb,Sr)(Nb,Zr,Ti)O₃,

(Pb,Mg)(Nb, Zr,Ti)O₃,

PZT ceramics exhibit piezoelectric effects over a wide range ofcompositions. The highest electromechanical coupling factors occur inthe region of the morphotropic phase boundary between the tetragonalferroelectric phase and the rhombohedral ferroelectric phase. Thisboundary occurs at approximately 48 mol % PbTiO₃ in the PbTiO₃-PbZrO₃system (see Piezoelectric Ceramics, B. Jaffe, W. A. R. Cook Jr., and H.Jaffe, , New York: Academic Press (1971)). Recent work with thin filmshas shown similar results (see “Measurement of PiezoelectricCoefficients of Ferroelectric Thin Films,” K. Lefki and G. J. M.Dormans, J. Appl. Phys. 76 (1994): 1764). Useful Zr/Ti ratios near themorphotropic boundary are in the range of from about 40/60 to about60/40.

MOCVD of oxide films is usually carried out by injecting a metalorganicprecursor gas stream into a deposition chamber at reduced pressure. Theprecursors decompose thermally on a heated substrate and are convertedto oxides by an oxidizing gas which is also fed into the chamber. Thepresent invention preferably is carried out with an MOCVD techniquepermitting the use of a wide range of safe precursor chemistry withoutthe control problems of traditional MOCVD methods, as more fullydescribed in “MOCVD of BaSrTiO3 for DRAMs”, P. S. Kirlin, Proc. of the4th ISIF, 1994; “Liquid delivery of low vapor pressure MOCVDprecursors”, Gardiner, R. A., Van Buskirk, P. C., Kirlin, P. S.,Proceedings Mat. Res. Soc. 1993 Fall Meeting, and U.S. Pat. No.5,204,314, issued Apr. 20, 1993, the disclosures of which hereby areincorporated herein by reference in their entirety.

In the liquid delivery MOCVD approach, liquid solutions are supplied toa vaporization zone using a high precision pump or other supplyarrangement, for example as shown in FIG. 3.

FIG. 3 is a schematic representation of a liquid delivery system 60 forsimultaneous, controlled introduction of metalorganic compounds to a CVDreactor. The metalorganic compounds are in respective liquid solutionsin reservoirs 50, 52 and 54 which are manifolded together by manifold 56joined in liquid flow communication with liquid precursor stream feedline 58. From feed line 58, the precursor stream is flowed under theaction of pump 68 through line 70 to the vaporization zone 72.

The vaporization zone may contain a heated frit or other elevatedtemperature vaporization matrix, such as a screen, mesh, metal orceramic discs, etc., on which the liquid is rapidly vaporized to formthe precursor vapor stream which then is flowed in vapor feed line 74 tothe CVD reactor 76 for deposition of the modified PZT film on asubstrate therein (not shown).

The major advantage of this liquid delivery MOCVD approach is thatliquids can be mixed just prior to vaporization; besides its simplicity,the ability to make slight composition adjustments via computer control(e.g, by metering the respective precursor liquids from the reservoirsto the pump and/or vaporization zone) makes this approach highlydesirable. This technique has been applied to the controlled depositionof PbLaTiO₃ (“Chemical Vapor Deposition of Pb_(1−x)La_(x)TiO₃, ” P. C.Van Buskirk, J. F. Roeder, S. Bilodeau, S. Pombrik, and H. Beratan,Integrated Ferroelectrics 6 (1995): 141), YBaCuO (“In-Situ Growth ofC-Axis Oriented YBa₂Cu₃O₇ on Silicon with Composite Buffer Layers byPlasma Enhanced Metalorganic Chemical Vapor Deposition” Zhang, J.;Gardiner, R. A.; and Kirlin, P. S.; Mat. Res. Soc. Symp. Proc., Vol.275, (1992) 419), TlBaCaCuO (“Metalorganic chemical vapor deposition ofTl₂Ba₂CaCu₂O_(x) superconducting thin films using fluorinatedβ-diketonate source reagents.” Hamaguchi, N.; Gardiner, R.; Kirlin, P.S.; and Dye, R.; Hubbard, K. M.; Muenchausen, R. E.; Appl. Phys. Lett.,57 (1990) 2136), and BaTiO₃ (“Reduced Pressure MOCVD of HighlyCrystalline BaTiO₃ Thin Films”, Van Buskirk, P. C.; Gardiner, R. A.; andKirlin, P. S., Nutt, S.,_J. Mater. Res. 7(3) (1992) 542).

Compared to traditional MOCVD bubbler technology, liquid delivery allowsdeposition rates five and ten times higher, respectively, for PLZT andBaSrTiO₃ films (“Large area growth of PZT films by MOCVD”, Shimizu, M.,Fujimoto, M., Katayama, T., Shiosaki, T., Proceedings, 5^(th) Int. Symp.on Integ. Ferroelectrics, Colorado Springs, Colo., Apr. 19-21, 1993; and“SrTiO₃ thin films by MOCVD for 1 Gbit DRAM applications, Lesaicherre,P.-Y., Proceedings, 6^(th) Int. Symp. on Integ. Ferroelectrics,Monterey, Calif., Mar. 14-16, 1994). The disclosures of all of theforegoing references are incorporated herein by reference.

The solvent media used in the liquid delivery MOCVD process of theinvention may suitably comprise solvent compositions as disclosed inU.S. patent application Ser. No. 08/414,504 filed Mar. 31, 1995 in thenames of Robin A. Gardiner, et al., U.S. patent application Ser. No.08/484,654 filed Jun. 7, 1995 in the names of Robin A. Gardiner, et al.,and U.S. patent application Ser. No. [Docket 2771-167CIP2] filed Nov.20, 1997 in the names of Thomas H. Baum, et al., which are compatiblewith the specific metalorganic precursors used for forming the (Pb, Sr,Ca, Ba)ZrTiO₃ thin film materials and efficacious in the constituentliquid delivery and chemical vapor deposition process steps.

FIG. 4 is a schematic representation of a portion of a liquid deliveryMOCVD process system 100 that may be usefully employed for PZT filmgrowth. The liquid stream comprising multiple precursor solutionshomogeneously mixed together is introduced to the system in line 101,and passes to the liquid stream pump 102, from which it is dischargedinto feed line 104 containing flow control valve 106 and passes into thevaporization chamber 108.

The vaporization chamber 108 of the FIG. 4 system may be similarlyconstructed as described in connection with the FIG. 3 liquid deliveryMOCVD system. The vaporization chamber 108 receives a flow of argoncarrier gas from line 134 containing flow control valves and a bypassflow loop, as well as a mass flow controller. The resulting precursorvapor then flows in line 110 containing flow control valve 112 into theCVD reactor 114 containing heated substrate 116 on which the desired PZTthin film is deposited.

Also introduced into the CVD reactor is a flow of oxidizer gas in line140 formed by makeup oxygen gas from line 136 and makeup N₂O gas fromline 138, each containing suitable valving and mass flow controllers.Effluent gas from the CVD reactor 114 passes to the trap 118 throughflow lines including a main (fast pump) line containing flow controlvalve 120 and an auxiliary (slow pump) line containing flow controlvalve 122. A vaporizer bypass line 130 containing flow control valve 132therein interconnects the vaporizer with the trap.

The reactor 114 and the trap 118 are in flow communication with pump 128by an interconnecting line containing throttle valve 126, to impose aproper pressure drop in the system and to ensure discharge of spentprecursor vapor from the CVD reactor.

A suitable lead precursor for MOCVD formation of lead-containingtitanates of the invention is leadbis(2,2,6,6-tetramethyl-3,5-heptanedionate) [Pb(thd)₂] and Lewis basecoordinated analogs. As used herein, the termthd=2,2,6,6-tetramethyl-3,5-heptanedionate. The Pb(thd)₂ compound has noappreciable vapor pressure at room temperature, which makes it muchsafer to handle than alkyl lead reagents such as tetraethyl Pb. However,the low volatility of Pb(thd)₂ (0.05 torr at 180° C.) requires the useof the method of liquid precursor delivery.

Titanium bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate)[Ti(OiPr)₂(thd)₂] and zirconiumtetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate) [Zr(thd)₄] are usefullyemployed as Ti and Zr source reagents, respectively. Further, othermetal β-diketonate analogs may also be usefully employed in theinvention. A second source for Zr is zirconiumbis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate). Thesecompounds are extremely soluble in organic media and no detrimentalligand exchange is possible since the titanium atom is coordinativelysaturated (see “Nuclear Magnetic Resonance and Infrared Spectral Studieson Labile cis-Dialkoxy-bis(acetylacetonato) titanium(IV) Compounds.”Bradley, D. C.; Holloway, C. E. J. Chem. Soc. (A), 282 (1969)).

For doping with Sr, the Lewis base adduct of Sr (thd)₂ mayadvantageously be used, where the Lewis base is tetraglyme,tetramethylethylene diamine, pentamethyl diethylenetriamine.

For doping with Ca, the Lewis base adduct of Ca(thd)₂ may advantageouslybe used, where the Lewis base is tetraglyme, tetramethylethylenediamine, pentamethyl diethylenetriamine. For doping with Ba, the Lewisbase adduct of Ba (thd)₂ may advantageously be used, where the Lewisbase is tetraglyme, tetramethylethylene diamine, pentamethyldiethylenetriamine.

For doping with Nb, niobium tetrakis(isopropoxide)(2,2,6,6-tetramethyl-3,5-heptanedionate), Nb(O-i-Pr)₄(thd) may beusefully employed.

For doping with Ta, tantalum tetrakis(isopropoxide)(2,2,6,6-tetramethyl-3,5-heptanedionate), Ta(O-i-Pr)₄(thd) may beusefully employed.

Other suitable and corresponding reagents for Ca, Ba, Mg, and Ta may beemployed. Illustrative other species of metalorganic compounds andcomplexes are disclosed in the aforementioned U.S. patent applicationSer. No. 08/414,504 filed Mar. 31, 1995 in the names of Robin A.Gardiner, et al., U.S. patent application Ser. No. 08/484,654 filed Jun.7, 1995 in the names of Robin A. Gardiner, et al., and U.S. patentapplication Ser. No. [Docket 2771-167CIP2] filed Nov. 20, 1997 in thenames of Thomas H. Baum, et al., the disclosures of which are herebyincorporated herein by reference in their entirety.

Concerning the technological import of the methodology and films of theinvention, MEMS technology based on passive devices already exists inthe commercial marketplace. For example, Si based micromachinedaccelerometers are used in air bag sense/deployment systems inautomobiles. It is not unreasonable to state that this technology willhave a pervasive impact on society. A whole class of active devices hasawaited the development of high quality piezoelectric actuating films,and many more devices requiring them will be designed in the future. Themanufacturable process of the invention for producing high qualitypiezoelectric films enables the development of these active MEMSdevices.

In the area of optical devices, both geometric and spectral (orinterference) based devices can be based on the high qualitypiezoelectric films of the invention. The geometric devices can befurther subdivided into refractive approaches, such as a movablemicrolens array, or reflective, such as a movable micromirror array.Piezoelectric thin films of the present invention can be used to alter aresonant cavity in an etalon structure to detune the reflectance of thedevice. Such approach has the advantage of requiring relatively littledisplacement, so response times can be maximized.

Micropumps and microvalves can also be based on cantilever geometryusing high quality piezoelectric films of the present invention.Micropumps can be used for delivering doses of medication or to runhydraulic systems in a MEMS configuration. Microvalves are useful inthis scheme as well, or for acting as a servo on a large industrialvalve controlling flow in a process.

Examples of other devices based on thin film PZT materials includeultrasonic transducers and active vibration control devices. Ultrasonictransducers can be fabricated by MOCVD for high frequency applicationswhich allow greater spatial resolution for detecting small defects. Thefrequency range between 50-150 MHz is very difficult to produce bytraditional bulk ceramic methods. MOCVD is a thin film-forming processwith a deposition rate high enough to make these devices at the requiredthickness of between 13 and 40 μm thick. Such transducers can be used toevaluate near surface flaws in aircraft, in microelectronics, and inbiological applications.

Other potential applications of the MOCVD process to deposit PZT andPSZT extend beyond microelectromechanical devices. In addition topiezoelectric actuation, thin film PZT can be exploited based onnumerous related ferroelectric properties. For example, uncooledinfrared radiation detectors based on the pyroelectric effect is atechnology enabled by thin film PZT and related materials. Non-volatilememory devices based on the ferroelectric effect can be used in militaryapplications for radiation hardened data storage and commercially as areplacement for EEPROMs and flash memory.

The deposition process of the invention facilitates integration of Sidrive electronics with MEMS devices. MOCVD is usefully employed inmicroelectronic fabrication as a manufacturing technique for large areadeposition. The present invention provides a unique MOCVD process todeposit Sr-doped PZT, which has been shown to dramatically increasepiezoelectric response relative to bulk ceramics. High quality thinfilms of PSZT has been produced for the first time and the piezoelectriceffect, the key requirement for any of the aforementioned MEMSapplications, has been demonstrated.

The optimum composition of the PSZT film and use of electrode processesto minimize electrical leakage can be readily determined by the skilledartisan based on the disclosure herein. Microfabrication techniques canbe used in place of macroscopic approaches for making cantileverstructures of piezoelectric thin film materials in the broad practice ofthe present invention.

The features and advantages of the present invention are more fullyshown with reference to the following illustrative and non-limitingexamples.

EXAMPLE 1

Films between 2500 and 5000 Å in thickness were deposited on two inchdiameter TiO₂/Pt/TiO₂/Si₃N₄/Si wafers at deposition temperaturesconsistent with Si device integration (550° C.). First, a baselineseries of PZT films was deposited with Zr/Ti ratios between 43/57 and53/47. Then, Sr-modified PZT films were deposited near Zr/Ti˜53/47, withsubstitution of 4-11% of Sr for Pb. A small piece of each wafer wasremoved for electrical characterization; the balance was utilized forblanket top electrode deposition and piezoelectric characterization.Cantilever structures were formed by cleaving the wafers into smallstrips. Piezoelectric characterization was carried out by applying ACsignals of varying frequency to the cantilever structures and measuringresonant vibrational modes using a laser interferometer.

Crystallinity of all films was excellent and x-ray diffraction patternsroutinely showed the desired perovskite crystal structure. Dielectricconstants of the samples ranged from ˜250 to 800 and electrical leakagecurrents for films with Pt top electrodes were acceptable. The Srmodified PZT films (i.e., PSZT films) had lower capacitance compared tothe unmodified PZT films, which is an advantage from the viewpoint of RCtime constant contributions to response times for devices made fromPSZT. All films displayed ferroelectric behavior, with well-saturatedhysteresis loops. More importantly, the piezoelectric effect wasobserved in all of the samples. This is the first demonstration of d₃₁piezoelectric effects in PZT films deposited by metalorganic chemicalvapor deposition (MOCVD) and the first demonstration of thin filmdeposition of PSZT in the art by any method.

Three solutions, each containing Pb, Zr, and Ti in different ratiosbracketing the composition range of interest were mixed to the desiredsolution composition for a given run. A fourth solution, containing onlySr, was added to the three-way PZT mixture to produce Sr modified PZT.

The MOCVD reactor used in the foregoing illustrative deposition was aninverted vertical configuration, of a type as shown in FIG. 4 hereof.The wafer was located face down in a stagnant flow zone which yieldedgood lateral uniformity. The diameter of the reactor near the wafer was5 inches, and the reactor was configured for 2 inch wafers. The vesselwas made of fused quartz and the walls of the reactor were heated byconvection using hot air flowing through a second quartz tube coaxial tothe reactor tube (not shown) to prevent condensation of the precursorsas they traveled through the reactor to the substrate. For theseillustrative depositions, the reactor conditions were as follows:

Substrate temperature: 550° C. Bottom electrode: Si/TiO₂/Pt/TiO₂ Totalreactor pressure: 2 Torr Reactor wall temperature: >230° C. Ar flow(through precursor manifold): 100 sccm O₂ flow 450 sccm N₂O flow 450sccm Film Thickness 0.25-0.6 μm

First order requirements for a good ferroelectric film are controlledcomposition and formation of the perovskite crystal phase. In order toassess these characteristics for the deposited films, the composition ofthe aforementioned films was characterized by X-ray fluorescence (XRF),and crystallinity was assessed by x-ray diffraction (XRD).

Crystallinity was assessed by X-ray diffraction (XRD) with a RigakuDMAX/3 HFX diffractometer in the Bragg-Brentano geometry (Θ-2Θ). Cu Kαradiation was used in all experiments with a crystal monochrometerbetween the sample and the detector. Five multiple scans were run oneach sample at a scan rate of 0.1°/sec.

Capacitor structures were fabricated for electrical characterization byevaporation of 1000 Å thick Pt top electrodes through a shadow mask.Deposition of Pt was carried out at 200° C. by use of halogen lampheating. Electrical characterization was comprised of capacitance andleakage measurements. Small signal AC capacitance was measured on aKeithley 590 CV meter as a function of applied DC bias between + and−5V. Bias was swept from both directions to observe ferroelectricswitching. Zero bias capacitance values were also recorded. Electricalleakage was measured using a Hewlett-Packard 4850 SemiconductorParameter Analyzer as a function of time for all samples at various DCbias voltages. Leakage values reported here were all obtained at 30 to100 seconds, which generally represented the true DC conduction in thefilm/electrode structures (i.e, steady-state leakage values).

Ferroelectric characterization was carried out on a Radiant TechnologiesRT6000 ferroelectric test system. Ferroelectric hysteresis loops weremeasured using a saw tooth AC signal with maximum voltages between 12-17V.

In connection with the above-discussed characterization effort, a seriesof eight calibration runs was performed in order to establish therelationship of precursor solution composition to film composition.After this relationship was established, a number of PZT films weredeposited in the target composition range between 40/60 and 60/40 Zr/Tiratios. The use of the liquid delivery technique provided the precisecontrol of composition required to form films of PZT within a relativelynarrow composition range.

FIG. 5 shows the composition of PZT films deposited by the liquiddelivery method, in the region near the morphotropic phase boundarybetween rhombohedral and tetragonal PZT.

Once this series of PZT films was complete, attention was focused on theSr modification. PSZT films were deposited by substitutingSr(thd)₂•tetraglyme for a portion of Pb(thd)₂ in the precursor solution.By using deposition conditions similar to those for PZT, films wereproduced of the desired stoichiometry and the perovskite crystal phase.Sr shifts the room temperature morphotropic boundary toward the PbZrO₃side of the quaternary system, so film compositions with higher Zr/Tithan 52/48 were targeted.

FIG. 6 shows the compositions of PSZT films deposited in accordance withthe method of the invention, with compositions at the room temperaturemorphotropic boundary being indicated. The phase boundary shifts towardthe PbZrO₃ side of the system with increased Sr as indicated in thefigure.

The thickness and compositions of the resulting films are summarized inTable I.

TABLE I Compositions of films given in terms of the stoichiometricformula Pb_(x/100) Sr_((100−x)/100) (Zr_(y/100) Ti_((100−y)/100))O₃. RunThickness Composition (Pb + Sr = 100, Zr + Ti = 100) No. (Å) Pb Sr Ti Zr 9 2623 100 0 50 50 12 4643 100 0 44 56 13 5193 100 0 54 46 14 4650 1000 43 57 16 4622 100 0 43 57 17 4749 100 0 53 47 18 4073 96 4 51 49 204477 90 10 55 45 21 5560 89 11 66 34

XRD revealed excellent crystallinity for all of the PZT and PSZT filmslisted in Table I. A representative series of X-ray diffraction patternsfor PZT films in the composition range varying from Zr/Ti=44/56 to 54/46is shown in FIG. 7. All films showed similar mixed [100]+[111] preferredorientation.

All films therefore had preferred orientations, in contrast to randompolycrystalline material, in which [110] yields the strongest diffractedintensity. The [100] component of preferred orientation is likely due tostresses induced by mismatch in coefficient of thermal expansion betweenthe substrate (Si ˜3 ppm/° C.) and the PZT (˜6-10 ppm/° C.) whichresulted in a tensile film stress. The minimum energy configurationcauses the long axis [001] of the tetragonal cell to lie in the plane ofthe film. The [111] preferred orientation is likely due to the TiO₂layer at the surface of the substrate that was used in this case as partof the electrode stack. Ti-rich PbTiO₃ seed layers have [111] preferredorientation (see, for example, “Effects of the Utilization of a BufferLayer on the Growth of Pb(Zr,Ti)O₃ Thin Films by Metalorganic ChemicalVapor Deposition,” M. Shimizu, M. Sugiyama, H. Fujisawa, T. Hamano, T.Shiosaki, and K. Matshushige, J. Cryst. Growth 145 (1994): 226).

FIG. 8 shows XRD patterns of the PSZT films deposited by MOCVD inaccordance with the method of the invention, with preferred orientationsimilar to PZT being observed in all films. The small peak at 2θ=30.5°is due to an unidentified second phase. The presence of the second phasedid not cause any serious degradation of film properties, and can beeliminated by further optimization of the composition.

Small signal AC capacitance measurements revealed ferroelectricswitching in the films when the direction of DC bias sweep was changed.FIG. 9 shows small AC signal capacitance as a function of applied DCbias for a PZT film deposited by MOCVD, wherein electrode area was8×10⁻⁴ cm², and film thickness was 5193 Å. The shift in the maxima withdirection of bias sweep is characteristic of a ferroelectric film.

The peak in the capacitance curve corresponds to the maximum slope ofthe polarization-voltage hysteresis curve, which generally coincideswith the coercive voltage required to switch the ferroelectric domainsin the thin film (i.e., the voltages at which the hysteresis curvecrosses the abscissa).

Dielectric constants for the PSZT films were found to be lower thanthose for the PZT films. This was in contrast to bulk data, but theeffect related to the preferred orientation of the films. For bulkmaterials, ε₃₃ is increased with Sr substitution (in the notationε_(xy), ε refers to permittivity, x refers to the direction ofmeasurement, y refers to the crystallographic direction); however thethin film material is strongly textured away from this orientation. Thereduced dielectric constant for PSZT is an advantage with respect to theRC time constant of the film. This time constant contributes to theoverall switching speed of a device, which is highly important inapplications such as hardened optical systems.

Loss tangent was also examined as a function of composition. FIG. 10shows loss tangent as a function of composition for films deposited byMOCVD in accordance with the method of the invention, wherein PSZT filmsshowed lower losses with the exception of the film at Zr/Zr+Ti=0.67where high losses were expected based on the high Zr content.

Loss tangent is known to increase at high levels of Zr, which accountsfor the high value in the 5.2 atomic % Sr film. Loss tangent isgenerally related to the amount of energy expended in moving domainwalls during ferroelectric switching; low loss is generally desirable.These results indicate that Sr improved losses in PZT films.

All of the deposited films displayed saturated ferroelectric hysteresisloops. FIG. 11a shows ferroelectric hysteresis loops for representativefilms deposited by MOCVD in accordance with the method of the invention,wherein Zr/Ti ratios are indicated along with Sr content. All loops werewell saturated, demonstrating the ferroelectric effect in both PZT andPSZT.

The shape of the hysteresis loop changed with composition, a result thatis observed for many ferroelectric materials. The demonstration offerroelectric behavior in all films showed that the intrinsic quality ofthe PZT and PSZT thin films was high.

Eight wafers were used in deflection measurements: samples 9, 13, 14,16, 17, 18, 20, and 21 and the wafers were processed in several batches.Upper electrodes on wafers 9, 13 and 14 were sputtered Cr/Ni while theremaining wafers had evaporated Ti/Pt upper electrodes (with Ti incontact with the PZT, used as a precaution for obtaining good adhesion).

In order to avoid the effects attributable to leakage currents caused bythe Ti in the top electrodes, deflections induced by an AC signal weremeasured using a laser vibrometer as an alternative to DC deflectionmeasurements, since leakage had less of an effect in dynamicmeasurements. One end of each beam of material was glued to a base andthe laser spot was directed onto the free end of the beam. The amplitudeof the AC excitation voltage was 2.82 V on all samples. The excitationvoltage was swept through a range of frequencies to identify theresonances; narrower scans were done around particular peaks.

Resonances were observed on all samples, corresponding to the resonantmechanical modes of the silicon cantilever beam, demonstrating that thefilms were indeed piezoelectric. The PZT film was thin with respect tothe silicon beam, so the film thickness had a minimal effect on theresonant frequencies. It was noted that no vibrations would have beenmeasurable if the films were not piezoelectric. In most cases a whistlewas audible as the sample was swept through the resonant frequency.

FIG. 12 shows a fundamental resonance vibrational mode of a cantileverbeam from sample 13, wherein the horizontal axis corresponded toexcitation frequency for a sinusoidal AC wave of ±2.8 V, and thevertical axis was proportional to the amplitude of the vibration,clearly demonstrating the piezoelectric effect in films produced inaccordance with the method of the present invention.

It was expected that the lowest measurable frequency was the fundamentalbending mode. This was verified on wafer PSZT-21 by scanning the laserspot along the beam from the free end toward the clamped end. Theamplitude varied quadratically as a function of position, verifying thatthis was the fundamental mode. Table 2 lists the fundamental frequenciesand vibration amplitudes measured at resonance at the end of each beam.

TABLE 3 Resonance data determined by laser vibrometer measurements with2.82 V excitation voltage amplitude. Wafer Beam Thickness Freq Ampl no.no. (mm) (Hz) (Å) 9 1 380 1240 0.20 9 2 380 943 1.6 9 3 380 6930 1.2 132 130 922 6.2 14 3 60 784 3.1 14 4 380 982 0.02 16 1 380 3675 2.7 16 2380 1195 0.33 16 3 380 4998 0.76 17 2 85 424 1.1 17 3 85 524 1.2 18 1380 2454 0.16 18 2 380 2503 0.13 20 1 380 1167 0.13 21 2 380 1098 0.3821 3 380 1189 0.37

All of the illustrative films described above clearly demonstratedpiezoelectric effects. The liquid delivery MOCVD approach of theinvention demonstrated the enhancements of d₃₁ piezoelectric coefficientobserved by Sr substitution in bulk ceramic materials.

EXAMPLE 2

The lead precursor chosen was leadbis(2,2,6,6-tetramethyl-3,5-heptanedionate) [Pb(thd)₂]. This compoundhas no appreciable vapor pressure at room temperature, which makes itmuch safer to handle than tetra-alkyl lead reagents. However, the lowvolatility of Pb(thd)₂ (0.05 Torr at 180° C.) requires the use of liquidprecursor delivery. Titaniumbis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate)[Ti(O-i-Pr)2(thd)₂] was used as the titanium precursor. Zirconiumtetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate) [Zr(thd)₄] was used asthe Zr source reagent. These compounds are extremely soluble in organicmedia and no possible detrimental ligand exchange is possible since thetitanium atom is coordinatively saturated.

Nb doping in PZT films by MOCVD requires a niobium precursor that isthermally stable, easily transported into the gas phase, and chemicallycompatible with the PZT precursors in solution and during thermalvaporization. Although ligand exchange reactions often occur insolution, exchange mechanisms must be degenerate and thus notdetrimental. To this end we explored the use of niobiumtetrakis(isopropoxide) (2,2,6,6-tetramethyl-3,5-heptanedionate)[Nb(O-i-Pr)₄(thd)]. This reagent provided chemical compatibility insolution and during gas-phase transport of the entire chemical mixture.

The following process conditions were applied:

Substrate temperature 550˜610° C. Bottom electrode Pt/<barrier>/Si Totalreactor pressure 2˜10 Torr Reactor wall temperature ˜200° C. Carrier Arflow ˜100 sccm O₂ flow 450 sccm N₂O flow 0˜450 sccm Total reagent Molarconcentration 0.29 M Reagent solution flow rate 0.1˜0.2 ml/min

In a representative run, the film was deposited at 610° C. on Pt/MgO/Si.The pressure was 5 Torr, oxidizer flow was a mixture of 450 sccm O₂ and450 sccm N₂O, and the reagent flow rate was 0.2 ml/min for 40 minutes.XRF analysis gave the following thickness and composition data:

Thickness (μm) Pb (at. %) Nb (at. %) Zr (at. %) Ti (at. %) 0.34 49.7 0.428.0 21.9

The film has 0.4 at % Nb with a Zr/Ti ratio of 56/44. XRD patterns takenfrom this film showed all characteristics associated with single-phaseperovskite PNZT thin film or substrate layers (Pt or Si). These resultsconfirmed the growth of high quality PNZT thin films by theabove-described process conditions.

EXAMPLE 3

Thin film (Pb_(x)Ca_(1−x)) (Zr_(y) Ti_(1−y−z) Ta_(z)) was deposited byliquid source MOCVD using the precursors shown below.

Lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate), [Pb(thd)₂]

Calcium bis(2,2,6,6-tetramethyl-3,5-heptanedionate).tetraglyme,[Ca(thd)₂-tetraglyme]

Zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate), [Zr(thd)₄]

Titanium bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate),[Ti(O-i-Pr)2(thd)₂]

Tantalum tetrakis(isopropoxide)(2,2,6,6-tetramethyl-3,5-heptanedionate), [Ta(O-i-Pr)₄(thd)]

These precursors are extremely soluble in organic media and exhibit nodetrimental ligand exchange. The solvent can be that described in ATMI167, ATMI 167-CIP, ATMI 167-CIP2 patent applications.

The following MOCVD process conditions were used:

Substrate temperature: ˜565° C.

Bottom electrode: Ir/SiO₂/Si

Total reactor pressure: 1.2 Torr

Reactor wall temperature: ˜220° C.

Carrier Ar flow: 200 sccm

O2 flow: 500 sccm

N2O flow: 500 sccm

Total reagent concentration: 0.30 M

Reagent flow rate: 0.14 ml/min.

Deposition time: 1950 seconds

Solution compositions were chosen such that nominally 1% of the A sitesare occupied by Ca and nominally 0.2% of the B sites are occupied by Ta.

XRD patterns taken from these films showed all characteristicsassociated with single-phase perovskite Ca and or Ta doped PZT thinfilms or substrate layers (Ir or Si). These results confirmed the growthof high quality doped PZT.

Capacitor structures were fabricated for electrical characterization byevaporation of 1000 Å thick Pt top electrodes through a shadow mask.Deposition of Pt was carried out at 200° C. by use of halogen lampheating.

Ferroelectric characterization was carried out on a Radiant TechnologiesRT6000 ferroelectric test system. Ferroelectric hysteresis loops weremeasured using a saw tooth AC signal with maximum voltages between 2.5V.Measurement of leakage current was carried out using a Kiethley 6517electrometer using a staircase voltage ramp with 0.5V steps and 5s dwelltime at each voltage. Current density measured in this way has bothrelaxation and steady state leakage contributions.

FIG. 13 shows ferroelectric hysteresis loops for films deposited byMOCVD in accordance with the method of the invention, with a Zr/Ti ratioof 40/60 both with and without 1% Ca and 0.2% Ta substitution. Bothfilms show full saturation below 2.5V and high remanent polarization.FIG. 14 shows the leakage current versus voltage for PZT with Zr/Ti of40/60 and the same Zr/Ti with 1% Ca and 0.2% Ta substitution. Thesubstituted material has {fraction (1/7)}th the leakage current densityat 3V compared to material which was not substituted.

Thus, while the invention has been disclosed herein with respect toillustrative aspects, features and embodiments, it will be appreciatedthat the utility of the invention is not limited thereby, but ratherextends to and encompasses a wide variety of variations, modificationsand other embodiments which are to be accorded as being within thespirit and scope of the invention as hereinafter claimed.

What is claimed is:
 1. A method of forming on a substrate a modified Pb(Zr,Ti)O₃ perovskite crystal material thin film including crystal lattice A-sites and B-sites at least one of which is modified by the presence of a substituent selected front the group consisting of (i) A-site substituents consisting of Sr, Ca, Ba and Mg, and (ii) B-site substituents selected from the group consisting of Nb and Ta, comprising liquid delivery MOCVD of the thin film from metalorganic precursors of the metal components of the thin film.
 2. The method according to claim 1, wherein the metalorganic precursors comprise titanium bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate), titanium bis(tert-butoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate) or titanium oxo bis(2,2,6,6-tetramethyl-3,5-heptanedionate).
 3. The method according to claim 1, wherein the metalorganic precursors comprise zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate) or zirconium bis(isopropoxide) bis(2,2,6,6-tetramethyl-3,5-heptanedionate).
 4. The method according to claim 1, wherein the metalorganic precursors comprise lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate) and Lewis base analogs of this material.
 5. The method according to claim 1,wherein the Pb(Zr,Ti)O₃ perovskite crystal material comprises Sr or barium as an A-site substituent, and wherein the metalorganic precursors comprise strontium or barium bis(2,2,6,6-tetramethyl-3,5-heptanedionate) coordinated with Lewis bases such as tetraglyme, tetramethyl ethylenediamine and pentamethyl diethylenetriamine.
 6. The method according to claim 1, wherein the Pb(Zr,Ti)O₃ perovskite crystal material comprises Ca or magnesium as an A-site substituent, and wherein the metalorganic precursors comprise calcium or magnesium bis(2,2,6,6-tetramethyl-3,5-heptanedionate) coordinated with Lewis bases such as tetraglyme, tetramethyl ethylenediamine and pentamethyl diethylenetriamine.
 7. The method according to claim 1, wherein the metalorganic precursors comprise niobium tetrakis(isopropoxide)(2,2,6,6-tetramethyl-3,5-heptanedionate).
 8. The method according to claim 1, wherein the metalorganic precursors comprise tantalum tetrakis(isopropoxide)(2,2,6,6-tetramethyl-3,5-heptanedionate).
 9. The method according to claim 1, wherein the metalorganic precursors comprise at least one of titanium bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate); zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate); lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate); strontium bis(2,2,6,6-tetramethyl-3,5-heptanedionate).tetraglyme; and niobium tetrakis(isopropoxide)(2,2,6,6-tetramethyl-3,5-heptanedionate).
 10. The method according to claim 1, wherein the metalorganic precursors comprise titanium bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate); zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate); and lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate).
 11. The method according to claim 1, wherein the metalorganic precursors comprise titanium bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate); zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate); lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate); and at least one of strontium bis(2,2,6,6-tetramethyl-3,5-heptanedionate).tetraglyme and niobium tetrakis(isopropoxide)(2,2,6,6-tetramethyl-3,5-heptanedionate).
 12. The method according to claim 1, wherein the metalorganic precursors comprise titanium bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate); zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate); lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate); and at least one of calcium bis(2,2,6,6-tetramethyl-3,5-heptanedionate) tetraglyme and tantalum tetrakis(isopropoxide)(2,2,6,6-tetramethyl-3,5-heptanedionate).
 13. The method according to claim 1, wherein the modified Pb(Zr,Ti)O₃ perovskite crystal material thin film has a composition selected from the group consisting of: (Pb,Sr)(Zr,Ti)O₃ , (Pb,Ca)(Zr,Ti)O₃, (Pb,Ba)(Zr,Ti)O₃, (Pb,Mg)(Zr,Ti)O₃, (Pb)(Ta,Zr,Ti)O₃, (Pb,Ca)(Ta,Zr,Ti)O₃, (Pb,Sr)(Ta,Zr,Ti)O₃, (Pb,Ca)(Nb,Zr,Ti)O₃, and (Pb,Sr)(Nb,Zr,Ti)O₃.
 14. The method according to claim 1, wherein the Pb(Zr,Ti)O₃ perovskite crystal material comprises PSZT.
 15. The method according to claim 1, wherein the Pb(Zr,Ti)O₃ perovskite crystal material comprises a composition of the formula Pb_(x)Sr_((1−x))Zr_(y)Ti_((1−y))O₃, wherein Pb:Sr:Zr:Ti has a ratio x:(1−x):y:(1−y), where x has a value of from about 0.86 to about 0.93, and y has a value of from about 0.50 to about 0.60. 